The wrought, Ni—Cr—Mo (C-type) alloys are popular materials of construction throughout the chemical process industries. Their primary attributes are high resistance to the halogen acids, in particular hydrochloric, and high resistance to chloride-induced corrosion phenomena, such as pitting, crevice attack, and stress corrosion cracking. In contrast, the austenitic and duplex stainless steels exhibit poor resistance to the halogen acids and to chloride-induced phenomena.
The basic structure of the wrought, C-type alloys is face-centered cubic. This is also the structure of nickel, a ductile and reasonably corrosion-resistant metal, in which large quantities of useful elements, such as chromium and molybdenum, are soluble. Notably, nickel is used to stabilize the same structure in the austenitic stainless steels. The chromium contents of the C-type alloys range from about 15 to 25 wt. %, while their molybdenum contents range from about 12 to 20 wt. %. The primary function of chromium is to provide passivity in oxidizing acid solutions; this is also its main function in the stainless steels. Molybdenum greatly enhances the resistance of nickel to reducing acids, in particular hydrochloric, and increases the resistance to localized attack (pitting and crevice corrosion), perhaps because these forms of attack involve the local formation of hydrochloric acid. Molybdenum provides some strengthening to the solid solution, on account of its atomic size.
Optional minor element additions include iron and tungsten. The primary purpose of including iron is to lessen the cost of furnace charge Materials, during melting. Interestingly, in the most recently developed C-type alloys, iron has been relegated to the role of an impurity, to increase the solubility of other, more useful elements. Tungsten is sometimes used as a partial replacement for molybdenum. In fact, specific tungsten-to-molybdenum ratios have been shown to provide increased resistance to localized attack within certain C-type alloys (U.S. Pat. No. 4,533,414).
The compositions of the prior Ni—Cr—Mo alloys are given in Table 1. They are all derivatives of HASTELLOY C alloy, a cast material patented (U.S. Pat. No. 1,836,317) in the early nineteen thirties. In later years, between the nineteen forties and nineteen sixties, HASTELLOY C alloy was also produced in the form of wrought products. Castings of this alloy are still used today, under the ASTM designation CW-12MW.
In the nineteen sixties, advances in melting technology allowed greater control of minor elements, in particular carbon and silicon, which encourage sensitization of the Ni—Cr—Mo alloys during welding, through the precipitation of deleterious carbides and intermetallic phases. U.S. Pat. No. 3,203,792 describes a range of low carbon and low silicon Ni—Cr—Mo alloys. The commercial embodiment of that patent was developed and marketed as HASTELLOY C-276 alloy, which is still the most widely used alloy of this family.
To reduce further the tendency for deleterious phases to form, a tungsten-free, low-iron composition, designated HASTELLOY C-4 alloy, was developed and patented (U.S. Pat. No. 4,080,201), in the nineteen seventies.
HASTELLOY C-22 alloy (U.S. Pat. No. 4,533,414) was developed in the early nineteen eighties. It was designed to cope with a wider range of environments than C-276 alloy, and to possess enhanced resistance to chloride-induced pitting and crevice corrosion. Notably, its chromium content was significantly higher than that of C-276 alloy, and a specific molybdenum-to-tungsten ratio was found desirable.
In the late nineteen eighties and early nineteen nineties, two additional Ni—Cr—Mo alloys were introduced, their primary benefit being higher resistance to chloride-induced pitting. One of these (U.S. Pat. No. 4,906,437) was a high-chromium, low-tungsten, low-iron composition called Alloy 59, and the other (INCONEL 686 alloy) was a high-chromium derivative of C-276 alloy, with a low iron content.
The next two prior art alloys in Table 1, namely HASTELLOY C-2000 alloy (U.S. Pat. No. 6,280,540) and MAT-21 (U.S. Pat. No. 5,529,642), both of which were introduced in the mid-nineteen nineties, are unusual in that they contain small amounts of copper and tantalum, respectively. Both of these elements enhance the corrosion resistance of the Ni—Cr—Mo alloys. U.S. Pat. No. 5,529,642 teaches that tantalum levels of 1.1 to 3.5 wt. % in a nickel-chromium-molybdenum alloy improve corrosion resistance.
The Ni—Cr—Mo alloys are normally used in the solution annealed and water quenched condition. To maximize their corrosion resistance, the amounts of chromium, molybdenum, etc. added to the C-type alloys exceed their solubilities at room temperature. In fact, the alloys are metastable below their solution annealing temperatures (which range from about 1900° F. to 2100° F.). The extent of alloying is actually governed by the kinetics of second phase precipitation, the design principle being that the alloys should retain their solution annealed structures when water quenched, and should not suffer continuous grain boundary precipitation of deleterious second phases in weld heat-affected zones.
With regard to the types of second phase precipitate normally found in the C-type alloys, those observed in C-276 alloy are as follows:
1. At temperatures between 300° C. and 650° C., an ordered phase of the type A2B, or in this case Ni2(Mo,Cr), occurs by long-range ordering. The precipitation reaction is described as being homogeneous, with no preferential precipitation at the grain boundaries or twin boundaries. The reaction is slow at lower temperatures within this range; it has been established, for example, that it takes in excess of 38,000 hours for A2B to form in C-276 alloy at 425° C.
2. At temperatures above 650° C., three precipitate phases can nucleate heterogeneously at grain boundaries and twin boundaries. These are μ phase, M6C carbide, and P phase. μ phase is described as having a hexagonal crystal structure and an A7B6 stoichiometry. M6C has a diamond cubic crystal structure, and P phase has a tetragonal structure. It has been discovered that μ phase precipitates in C-276 alloy within the temperature range 760° C. to 1094° C., whereas M6C carbide precipitates at temperatures between 650° C. and 1038° C. It has also been found that that the kinetics of carbide formation are faster than those of μ phase.
As to the effects of these second phase precipitates on the properties of the C-type alloys, it is well known that the heterogeneous precipitates that occur at temperatures in excess of 650° C. are detrimental to both corrosion resistance and material ductility. On the other hand, previous work (described in U.S. Pat. No. 4,129,464) has shown that the homogeneous precipitation reaction (A2B ordering) that occurs at lower temperatures can be used to strengthen the C-type alloys, while maintaining good ductility. However, this reaction can lead to loss of corrosion resistance.
Although technically not a C-type alloy, the Ni—Mo—Cr based 242 alloy (U.S. Pat. No. 4,818,486) is also included in Table 1. This alloy was designed for high temperature, high strength applications, rather than for use in the chemical process industry. It is of relevance in this discussion because it derives its high strength from the same type of A2B ordering observed in C-type alloys. However, the age hardening treatment responsible for inducing this A2B ordering can be performed in 48 hours or less, a considerably shorter time than required for such ordering in C-type alloys. However, with only 8% Cr the 242 alloy is not well suited for many environments important in the chemical process industry.
Recently, a strengthening heat treatment was discovered which induces A2B ordering in C-type alloys in a relatively short time of 48 hours or less. This heat treatment was effective over a fairly wide range of Cr and Mo levels, but only when the overall composition was carefully controlled according to a specific numerical relationship. For many of the compositions, this two step heat treatment was effective in inducing strengthening where single step aging treatments would take significantly longer time. A heat treatment time of 48 hours or less is of definite importance in determining the commercial practicality of such a treatment. It was also discovered that, within the temperature range of the two step aging treatment, precipitation of deleterious phases does not appear to be significant, at least at the carbon contents normally encountered with the wrought C-type alloys. These discoveries were described in a recent U.S. Pat. No. 6,544,362 and in related U.S. Pat. Publication No. US-2003-0051783-A1.
Given this knowledge, the objective during development of the present invention was to determine a Ni—Cr—Mo composition which would not only respond to the strengthening heat treatment, but which would not significantly lose corrosion resistance upon receiving this heat treatment.
TABLE 1Nominal Compositions (Wt. %) Of Prior Art AlloysU.S. Pat. No.1,836,3173,203,7924,080,2014,533,4144,906,4375,019,1845,529,6426,280,5404,818,486ALLOYCC-276C-4C-2259686MAT-21C-2000242NiBALANCEBALANCEBALANCEBALANCEBALANCEBALANCEBALANCEBALANCEBALANCECr16.51616222321192325Mo17161613161619168W4.54—3—3.7———Fe5.755  3 max31.5 max  5 max  1 max  3 max  2 maxMn1 max  1 max  1 max 0.5 max0.5 max0.75 max 0.5 max 0.5 max0.8 maxTa——————1.85——Cu———————1.60.5 maxSi1 max0.08 max0.08 max0.08 max0.1 max0.08 max0.08 max0.08 max0.8 maxC0.12 max  0.01 max0.01 max0.01 max0.01 max 0.01 max0.015 max 0.01 max0.03 max V0.30.35 max—0.35 max——0.35 max——Ti—— 0.7 max——0.15———